1. Field of the Invention
This invention relates to the destruction of nitrogen oxide (NO.sub.x) compounds in a corona discharge pollutant destruction apparatus.
2. Description of the Related Art
Passing a pollutant bearing gas through a corona discharge site is a known method of removing the pollutants from the gas. A general review of this technique is provided in Puchkarev et al., "Toxic Gas Decomposition by Surface Discharge," Proceedings of the 1994 International Conf. on Plasma Science, Jun. 6-8, 1994, Santa Fe, N. Mex., paper No. 1E6, page 88. Corona pollutant destruction has also been proposed for liquids, as disclosed in application Ser. No. 08/295,959, filed Aug. 25, 1994, now U.S. Pat. No. 5,549,795, "Corona Source for Producing Corona Discharge and Fluid Waste Treatment with Corona Discharge," and assigned to Hughes Aircraft Company, now doing business as Hughes Electronics.
In one system, described in Yamamoto et al., "Decomposition of Volatile Organic Compounds by a Packed Bed Reactor and a Pulsed-Corona Plasma Reactor," Non-Thermal Plasma Techniques for Pollution Control, NATO ASI Series Vol. G34 Part B, Ed. by B. M. Penetrante and S. E. Schultheis, Springer-Verlag Berlin Heidelberg, 1993, pages 87-89, brief high voltage pulses of about 120-130 nanoseconds duration are applied to the center conductor of a coaxial corona reactor through which gas is flowing. Each pulse produces a corona discharge that emanates from the center wire and floods the inside volume of the reactor with energetic electrons at about 5-10 keV. A similar system is described in U.S. Pat. No. 4,695,358, in which pulses of positive DC voltage are superimposed upon a DC bias voltage to generate a streamer corona for removing SO.sub.x and NO.sub.x from a gas stream. These processes have relatively poor energy efficiencies. With the reactor geometries that have been selected, it is necessary to deliver very short pulses to avoid arc breakdown between the electrodes, and consequent damage. The pulse-forming circuit loses approximately half of the power coming from a high voltage supply in a charging resistor, and additional energy is wasted in a double spark gap. Furthermore, the capacitive load of the coaxial corona reactor must be charged; this charging energy is typically much greater than the energy that is actually used in the corona reaction, and simply decays away into heat after each pulse without contributing to the pollutant destruction.
A similar approach, but with a different reactor geometry, is taken in Rosocha et al., "Treatment of Hazardous Organic Wastes Using Silent-Discharge Plasmas," Non-Thermal Plasma Techniques for Pollution Control, NATO ASI Series Vol. G34 Part B, Ed. by B. M. Penetrante and S. E. Schultheis, Springer-Verlag Berlin Heidelberg, 1993, pages 79-80, in which the corona discharge is established between parallel plates. This system also suffers from a poor specific energy due to inefficient pulse formation and non-recovery of reactor charging energy.
A block diagram of a generic single-stage corona discharge pollutant destruction apparatus is shown in FIG. 1. A corona discharge reactor 2 takes pollutant-bearing exhaust gas 12 from an engine 6 through an inlet conduit 8, treats the exhaust gas, and discharges the treated exhaust gas 14 through an outlet conduit 10. Major pollutants in the exhaust gas 12 from the engine 6 include various forms of nitrogen oxides (NO.sub.x), hydrocarbons (HC), and carbon monoxide (CO). HC and CO are considered high energy level pollutants, which can be oxidized to produce water (H.sub.2 O) and carbon dioxide (CO.sub.2). NO.sub.x compounds are considered low energy level pollutants, and need to absorb energy to be reduced to harmless diatomic nitrogen (N.sub.2) and oxygen (O.sub.2). When a power source 4 supplies high voltage pulses to the corona discharge reactor 2, HCs are oxidized to become H.sub.2 O and CO.sub.2, while CO is oxidized to become CO.sub.2. At each voltage peak, corona charges are emitted within the reactor 2, producing free radicals that oxidize HC to generate CO.sub.2 and H.sub.2 O and CO to generate CO.sub.2. In general, high voltage pulses are very effective in destroying HC and CO, but have not been shown to be effective in the reduction of NO.sub.x into N.sub.2 and O.sub.2. Experiments have shown that corona generation using high voltages (up to 12 kV) may even produce some additional NO.sub.x. On the other hand, low voltage pulses are highly efficient in reducing NO.sub.x, but are very poor at oxidizing HC. The corona discharge process has achieved limited success in destroying NO.sub.x, which include NO and NO.sub.2, in the presence of oxygen and water. A problem with corona discharge is that it produces a strong oxidizing atmosphere by generating ozone (O.sub.3) and radicals such as O and OH. The free radicals and O.sub.3 are highly reactive oxidizers and react with both NO and N.sub.2 to produce NO.sub.2.
Injection of either hydrocarbon additives or ammonia (NH.sub.3) is a known method for substantial reduction of NO.sub.x compounds in corona discharge reactors. A general review of injecting NH.sub.3 or hydrocarbon additives is provided in G. E. Vogtlin and B. M. Penetrante, "Pulsed Corona Discharge for Removal of NO.sub.x from Flue Gas," Non-Thermal Plasma Techniques for Pollution Control, NATO ASI Series Vol. G34, Part B, page 187, 1993. If NH.sub.3 is injected, it reacts with acids formed in the reactor to produce ammonia salts, which are then collected by either a filtration system or other particulate removal system. A disadvantage of this method is that accumulated solid ammonia salts must be removed periodically, and thus is inconvenient for automotive applications. Reduction of NO.sub.x by adding NH.sub.3 in a corona discharge reactor has been achieved only at stationary source combustion sites. NH.sub.3 has not been used for NO.sub.x reduction in the treatment of automobile internal combustion engine exhaust because of hardware manufacturing and operating cost. Injection of hydrocarbon additives has been shown to be effective by recycling the hydroxyl radicals (OH) during the oxidation and reduction of NO. The efficiency of OH radical recycling is dependent on the reaction rate of the hydrocarbon additive with the OH radicals, and is described in G. E. Vogtlin et al.
The injection of engine oil in a plasma reactor to treat diesel engine exhaust is described in M. Higashi et al., "Soot Elimination and NO.sub.x and SO.sub.x Reduction in Diesel-Engine Exhaust by a Combination of Discharge Plasma and Oil Dynamics," IEEE Transaction on Plasma Science, Vol. 20, No. 1, 1992. The article reported the removal of NO.sub.x from diesel engine exhaust by the use of a discharge plasma in a reactor injected with drops of engine oil. Once the engine oil is in the chamber, the plasma generates a fine oil mist which stimulates the reduction of soot as well as NO.sub.x and SO.sub.x compounds.